Partially address #1409: group with named tuples

[piever]

This allows grouping with NamedTuples to give a name to the entries in the legend. For example:

julia> using Plots

julia> using NamedTuples

julia> plot(rand(10), group = @NT(a = rand(Bool,10), b = rand(Bool, 10)))

In #1409 this would allow:

@df df plot(:R, :F, group = @NT(L = :L, T = :T), legend=:topleft, marker=:circle, frame=:box, grid=false)

[carstenbauer]

Maybe put a comma, i.e. a = false, b = true (I forgot this in #1409). Otherwise this looks great! Thanks so much.